1. Field of the Invention
The present invention relates to a system for generating integrated circuit patterns or the like on photomasks or semiconductor wafers coated with photoresist, using laser radiation. The invention uses plural laser beams in a coaligned, closely spaced noninterfering array. The beams are concurrently deflected across a region of the target surface by an acousto-optic deflector driven by a swept frequency drive signal. The target is moved perpendicular to such angular deflection to position it for exposure of the next adjacent portion of the pattern. The extent of target offset in the direction of deflection is measured and used to control the initiation of modulation during each deflection stroke so that each generated portion of the pattern begins from a uniform reference line on the target. Pattern data supplied to the beam modulators is timed by data clock pulses established by the rate of change of frequency of the deflector drive signal.
2. Description of the Prior Art
The production of an integrated circuit normally begins with a photomask, which is a photographic negative of a layer of the circuit. The photomask for the first layer of the integrated circuit is projected onto a wafer of silicon which is coated with a photosensitive material. The latent image of the circuit pattern for the first layer is then developed, and the silicon uncovered in this process is appropriately treated to change its electrical characteristics. The steps are repeated for each circuit layer using an appropriate photomask.
This microlithographic process was initially performed using contact printing, wherein the photomask was brought into physical contact with the wafer. Contact printing was later supplanted by one-to-one projection printing of the circuit onto the photoresist material. More recently, 10X reticles have been used. These reticles are photomasks of one layer of an integrated circuit pattern enlarged ten times and produced on a glass plate. A ten-to-one reduced image of this 10X reticle is projected onto the wafer to expose the photoresist according to the circuit pattern on the reticle. The use of 10X reticles reduces the effect of defects in the photomask and has enabled smaller patterns to be placed on the wafer then had previously been possible.
Usually the photomasks are produced by photographic reduction of hand or computer generated artwork that is many times larger than the article itself. More recent techniques for producing the photomasks and the 10X reticles include the use of a raster-scanned electron beam. The electron beam is selectively turned on and off during the raster-scan to generate the required pattern. The procedure is typically aided by a computer. Because of the substantial amount of information necessary to create a detailed integrated circuit pattern, this e-beam scanning procedure is time consuming and expensive.
Attempts have been made to use a single raster scanned laser beam to generate the integrated circuit pattern on the reticle. The achievable minimum circuit detail is comparable to that of an electron beam system. Scanning time is very substantial, making on-wafer pattern generation both time consuming and expensive. Moreover, it is difficult to maintain sweep linearity across the entire wafer or reticle, and equally or more difficult to maintain close parallel alignment of adjacent scan lines. For these reasons, a commercially practical single beam laser pattern generator has not been achieved in the past.
An objective of the present invention is to provide a laser pattern generator which overcomes these disadvantages. A further objective is to provide a laser pattern generator in which plural closely spaced, noninterfering beams are simultaneously deflected and modulated so as to accomplish pattern generation rapidly and with requisite beam positioning and modulation control so as to obtain very fine pattern resolution. The invention is usable both for reticle production and for direct on-wafer pattern generation.